1. Field of the Invention:
The present invention relates generally to optical scanners for use in laser printers, for example, and more particularly to a post-objective type optical scanner including an optical deflector for scanning a beam after it is converged by a condensing lens, and also to an image forming apparatus using such post-objective type optical scanner.
2. Description of the Prior Art:
Most of the conventional optical scanners used in laser printers are of the pre-objective type. The pre-objective type optical scanners include a condensing lens disposed downstream of an optical deflector, so that the curvature of field or the f.theta. characteristic can be easily compensated by means of the condensing lens. The pre-objective type optical scanners, however, require a large condensing lens and are complicated in construction and hence costly to manufacture. Furthermore, since the pre-objective type optical scanners need a long optical path which makes it difficult to minimize the overall size of the optical scanners. In recent years, post-objective type optical scanners including a condensing lens disposed upstream of an optical deflector have been put into the research development and practical use for the purpose of realizing a compact and low-cost optical scanner.
One example of such conventional post-objective type optical scanners is disclosed in Japanese Patent Laid-open Publication No. 1-169422. In the disclosed post-objective type optical scanner, the reflecting surface of an optical deflector is composed of a cylindrical surface or a spherical surface so as to compensate the curvature of field in the scanning direction. Further, a compensating lens disposed between the optical deflector and a scanning surface has a power in the sub-scanning direction which varies in the scanning direction between the center and the periphery so as to compensate the curvature of field in the subscanning direction. With this construction, however, due to the aberration caused, it is difficult to form, over the entire area of the scanning surface, a spot size with an intensity of 1/e.sup.2 (e=base of natural logarithm) which is not exceeding 80.mu.m. With this difficulty, the resolution of the known post-objective type optical scanner is a relatively low.